High strength polycrystalline ceramic spheres

ABSTRACT

A method for making hollow spheres of alumina or aluminate comprises: coating polymeric beads with an aqueous solution of an alumoxane, drying the beads so as to form an alumoxane coating on the beads; heating the beads to a first temperature that is sufficient to convert the alumoxane coating to an amorphous alumina or aluminate coating and is not sufficient to decompose the polymeric beads; dissolving the polymeric bead in a solvent; removing the dissolved polymer from the amorphous alumina or aluminate coating; and heating the amorphous alumina or aluminate coating to a second temperature that is sufficient to form a hollow ceramic sphere of desired porosity and strength. The hollow spheres can be used as proppants or can be incorporated in porous membranes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of U.S. patent application Ser. No.10/774,319, filed Feb. 6, 2004 and entitled “High StrengthPolycrystalline Ceramic Spheres,” which claims the benefit of U.S.Provisional Application Ser. No. 60/445,551, entitled “High StrengthPolycrystalline Ceramic Spheres,” and U.S. Provisional Application Ser.No. 60/445,586, entitled “Method of Making Hierarchical CeramicUltrafiltration Membranes,” both filed Feb. 6, 2003. All of whichapplications are hereby incorporated by reference herein in theirentirety.

SPONSORED RESEARCH OR DEVELOPMENT

Research leading to the present invention was supported in part by thefederal government under Grant No. DMI-9613068 awarded by the NationalScience Foundation. The United States government may have certain rightsin the invention.

BACKGROUND OF THE INVENTION

Recognition that the macroscopic properties of materials depend not onlyon their chemical composition, but also on the size, shape andstructure, has spawned investigations into the control of theseparameters for various materials. In this regard, the fabrication ofuniform hollow spheres has recently gained much interest. Hollowcapsules with nanometer and micrometer dimensions offer a diverse rangeof potential applications, including: utilization as encapsulants forthe controlled release of a variety of substances, such as drugs, dyes,proteins, and cosmetics. When used as fillers for coatings, composites,insulating materials or pigments, hollow spheres provide advantages overthe traditional solid particles because of their associated lowdensities. Hollow spheres may also be used in applications as diverse ashierarchical filtration membranes and proppants to prop open fracturesin subterranean formations. A spherical morphology also allows forapplications in optical devices.

The geometry of the spheres has been shown to increase the strength ofcomposite materials. Incorporating hollow spheres into compositematerials improves the strength and the fracture strength of thematerial. Typically, materials (organic or inorganic) are reinforcedwith fibers that retard the propagation of stress cracks. When hollowparticles are incorporated into the fiber-reinforced composite, thecrack growth is further stopped by the neighboring particles, forexample, incorporation of glass beads into an epoxy resin.

Hollow particles have been fabricated from a variety of materials, suchas polymers, metal, ceramics, and glass, however, a great deal ofresearch has focused on various metal oxides, due to their chemical,thermal, and oxidative resistance, and because they have low dielectricconstants and are optically transparent. Conventional methods to producehollow ceramic spheres are vapor deposition, sputtering, molecular beamdeposition and electrolytic deposition; however, these processes do notalways provide a uniform coating of individual particles. Ceramicspheres exhibiting a uniform coating and thickness have been achievedwith the sol-gel route. Typically the spheres are formed by templatingwith either polystyrene spheres or silica spheres. The polystyrene orsilica spheres are coated with the sol-gel, after which the core isetched away, and calicination results in a ceramic hollow sphere.Titanium dioxide, barium titanate, alumina, and aluminosilicate sphereshave been fabricated using the sol-gel templating technique.

It has previously been shown that for alumina films and bodies, a lowcost, flexible, alternative to sol-gels are chemically functionalizedalumina nanoparticles (carboxylate-alumoxanes) (R. L. Callender, C. J.Harlan, N. M. Shapiro, C. D. Jones, D. L. Callahan, M. R. Wiesner, R.Cook, and A. R. Barron, Chem. Mater. 9 (1997) 2418, incorporated hereinby reference). These alumina nanoparticles may be prepared, in the sizerange 10-100 nm with a narrow size distribution, by the reaction of themineral boehmite with a wide range of carboxylic acids. Besides the useof aqueous reaction conditions, without mineral acids or other additives(resulting in high ceramic yields and low shrinkage), thecarboxylate-alumoxanes are stable in solution or the solid state (i.e.,they do not precipitate or undergo changes in particle size ordinarilyassociated with aging of sol-gels). Carboxylate-alumoxanes may be usedas ceramic precursors for the coating on carbon, SiC and Kevlar fibers(as demonstrated in R. L. Callender and A. R. Barron, J. Mater. Res. 15(2000) 2228, herein incorporated by reference).

A further advantage of the carboxylate-alumoxane nanoparticle approachis that the porosity of the ceramic formed upon thermolysis may becontrolled by the substituent of the carboxylic acid, which has led totheir application as precursors for ceramic membranes. A final advantageof the alumoxane approach over traditional sol-gel is the ease by whichaluminate phases may be prepared, often at a lower temperature thanpreviously observed. Thus, carboxylate-alumoxanes may be used to createhollow spheres of alumina or an aluminate. Furthermore, it haspreviously been shown that layer-by-layer (LbL) growth of laminates ispossible, which opens-up the possible fabrication of ceramic compositeswith increased applications, such as the formation of magnetic materials(Z. Y. Zhong, T. Prozorov, I. Felner, and A. Gedanken, J. Phys. Chem.103 (1999) 947, herein incorporated by reference).

Alumina sol-gel-derived membranes are presently the most accepted routesto making alumina ultrafiltration filters. Lennears, Keizer, andBurgraff first developed the technique of using sol-gel processes tomake alumina ultrafiltration membranes. These filters, along with thevast majority of those reported in the literature, were made by thecontrolled hydrolysis of aluminum alkoxides to form alumina. Thepreparation techniques used by various researchers vary the drying orsintering conditions, which result in small changes in porosity or poresize. The membrane selectivity is primarily dependent upon the pore-sizedistribution; the narrower the pore size distribution, the moreselective the membrane. However, for membranes produced by sol-geltechniques the pore size is generally limited to the size distributionof the precursor particles before sintering, which is difficult tocontrol. Furthermore, sol-gels must be used immediately afterpreparation to avoid aggregation or precipitation.

It has previously been reported that the fabrication of asymmetricalumina ultrafiltration membranes may be accomplished using carboxylicacid surface stabilized alumina nanoparticles (carboxylate-alumoxanes)(D. A. Bailey, C. D. Jones, A. R. Barron, and M. R. Wiesner,“Characterization of alumoxane-derived ceramic membranes”, J. MembraneSci., 176, (2000), 1-9 and C. D. Jones, M. Fidalgo, M. R. Wiesner, andA. R. Barron, “Alumina ultrafiltration membranes derived fromcarboxylate-alumoxane nanoparticles”, J. Membrane Sci., 193, (2001),175-184, herein incorporated by reference). A comparison with membranesderived from sol-gel methods showed carboxylatealumoxane-based membraneproperties to be favorable. The synthesis of the alumina nanoparticlesis simple and low cost, producing a defect free membrane in a one-stepprocess. For example, the carboxylate-alumoxane nanoparticles may beprepared in sufficient quantity for 100-200 m² of finished membrane in asingle laboratory-scale batch, with the cost of raw material being lessthat $5. Once prepared, the carboxylate-alumoxanes are stable for monthsin solution, or may be dried and redissolved as desired, without changesin particle size.

For both sol-gel and the present carboxylate-alumoxane derivedmembranes, mechanical integrity and permeability may be enhanced bysupporting a relatively thin selective membrane on a thicker, morepermeable substrate so as to produce an asymmetric membrane. Despitethis approach, and due to the small pore size of the alumoxane-derivedmembranes, the permeability of the asymmetric membranes is significantlylower than that of the support. In order to approach the permeability ofthe support, the ultrafiltration membrane must be as thin as possible.Sol-gel derived membranes often require multiple dip-fire sequences toensure integrity. In contrast, a single step process is sufficient forthe alumoxane nanoparticle approach. Unfortunately, in order to ensure adefect free membrane, a thickness of 1-2 μm is required. Thus, analternative approach must be used in both processes in order to increaseflow.

A typical technique for constructing a sol-gel membrane is to layermaterials of different porosity such that the thinnest possible layer ofthe “effective” ultrafiltration membrane is provided. However, this alsorequired multiple process steps and each layer may result in a decreasein flux. If decreasing membrane thickness is not practical, analternative approach is to increase the macroscopic surface area of themembrane. By analogy with biological membranes, one proposal is thecreation of a hierarchical structure, in which the macroscopic structureevolves through ever decreasing sizes. A good example of such astructure would be the mammalian lung. A hierarchical approach haspreviously been used for organic membranes and mesoporous materials (seeT. Kawasaki, M. Tokuhiro, N. Kimizuka, T. Kunitake, “Hierarchicalself-assembly of chiral complementary hydrogen-bond networks in water”,J. Am. Chem. Soc., 123 (2001) 6792-6800 and H.-P. Lin, Y.-R. Cheng,C.-Y. Mou, “Hierarchical order in hollow spheres of mesoporoussilicates”, Chem. Mater. 10 (1998) 3772-377, incorporated herein byreference).

SUMMARY OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention comprise polycrystalline aluminaand aluminate hollow spheres with a hardness approaching crystallinesapphire as well as applications utilizing such spheres. One suchapplication is the use of hollow spheres in filtration membranes. Theapplication of ceramic membranes in pollution prevention, resourcerecovery and waste treatment activities is increasing due to theirexcellent mechanical strength and tolerance to solvents, as well as toextremes of pH, oxidation, and temperature. In addition, the amphotericproperties of ceramic surfaces result in uniquely versatile membranesfor water and waste-water treatment.

Embodiments of the present invention also comprise methods forfabrication of hollow alumina and aluminate spheres. The steps in suchmethods comprise coating a polystyrene bead with an alumoxane solution,drying the bead, and then heating the coated bead to a temperaturesufficient to calcine the alumoxane to porous amorphous alumina. Thecoated bead is then washed in a solvent to remove the polystyrene beadfrom inside the coating. The remaining shell is then heated to atemperature sufficient to form an α-alumina sphere.

Further embodiments of the present invention include the methods andapparatus to use hollow alumina or aluminate spheres in applicationssuch as proppants, composites, and filtration membranes.

Still further embodiments of the present invention comprise a method forproducing a ceramic filtration membrane by suspending polystyrene beadsin an aqueous alumoxane solution. In these embodiments, an α-aluminasupport is then coated in the polystyrene/alumoxane colloidal solution,dried, and heated to a temperature sufficient to cause the top of thecoated spheres to burst

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed understanding of the preferred embodiments,reference is made to the accompanying Figures, wherein:

FIG. 1 is a schematic diagram of the process for forming hollow aluminaspheres.

FIG. 2 is a scanning electron microscope (SEM) image of dry-formpolystyrene beads.

FIG. 3 is an SEM image of coated beads.

FIG. 4 is an SEM image of coated beads.

FIG. 5 is an SEM image of beads after washing with solvent.

FIG. 6 is an SEM image of beads after heating to 1000° C.

FIG. 7 is a table summarizing the pore volume and surface area forsamples at each stage of the synthesis.

FIG. 8 is an SEM image of a wall of an alumina sphere.

FIG. 9 is chart comparing the hardness of various components.

FIG. 10 is an SEM image of a sphere exhibiting plate like crystals.

FIG. 11 is a schematic representation of an asymmetric filtrationmembrane.

FIG. 12 is a schematic representation of a hierarchical filtrationmembrane.

FIG. 13 is a table summarizing properties of various components.

FIG. 14 is a schematic representation of a hierarchical filtrationmembrane.

FIG. 15 is an SEM image of an asymmetric membrane.

FIG. 16 is an SEM image of an asymmetric membrane.

FIG. 17 is an SEM image of an asymmetric membrane.

FIG. 18 is an SEM image of an asymmetric membrane.

FIG. 19 is an SEM image of an asymmetric membrane.

FIG. 20 is a table summarizing properties of various membranes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A schematic representation of the process for forming hollow aluminaspheres is shown in FIG. 1. In general, a plurality of beads 10 arecoated with an alumoxane coating 20, which is then fired so that itbecomes a porous amorphous alumina layer 30. The beads are dissolved andremoved, leaving a hollow shell of porous amorphous alumina, which isthen fired so that it becomes a hard ceramic shell 40. These steps areillustrated in greater detail in the Examples below.

EXAMPLES

Hollow Spheres

An acetate-alumoxane (A-alumoxane) (not shown) was prepared according tothe method described in Chem. Mater. 9 (1997) 2418 by R. L. Callender,C. J. Harlan, N. M. Shapiro, C. D. Jones, D. L. Callahan, M. R. Wiesner,R. Cook, and A. R. Barron, which is incorporated herein by reference.Aqueous solutions of alumoxane were then degassed before use. Dry-formpolystyrene beads 10, such as those available from Polysciences, Inc.and shown in FIG. 2, were preferably used. Beads of polymers other thanpolystyrene may be used, so long as the polymer is soluble in a solvent.Likewise, beads of other materials may be used, so long as they aresoluble in a solvent that will not damage the alumoxane coating.

Beads 10 were then coated with the aqueous solution of A-alumoxane, asshown in the SEM image in FIG. 3. The aqueous solution of A-alumoxanemay range from 1-10 weight percent. The aqueous solution of A-alumoxanemore preferably ranges from 2-8 weight percent, and most preferably is 8weight percent. Beads 10 may range from 1-80 μm in diameter, and arepreferably 1-5 μm in diameter and more preferably about 3 μm indiameter.

The solution was pipetted onto beads 10 that were placed in a coatedceramic firing crucible (not shown), and allowed to dry in air. Thecoating process was conducted in a ceramic firing crucible to minimizethe amount of agitation of beads 10. Beads 10 were covered or coatedthree times to achieve a uniform alumoxane coating 20. The number ofcoatings is important in obtaining a structurally sound alumina sphere.It was found that three coating/drying cycles were preferred to providean alumoxane shell with good shape retention and uniformity. If a singlecoating/drying cycle was used, the spheres collapsed upon firing to1000° C.

Alumoxane-coated polystyrene beads 10 were fired to 220° C. for 40minutes to burn off organic substituents (not shown). The firingconverted the alumoxane coating 20 to a porous amorphous alumina coating30, as shown in the SEM image in FIG. 4. This allows a solvent such astoluene (not shown) to dissolve polystyrene beads 10 but not amorphousalumina coating 30. Beads 10 with amorphous alumina coating 30 werestirred in toluene for 1 hour and then vacuum filtered, with the resultsshown in the SEM image in FIG. 5. The washing process was performed 5times before firing to 1000° C. to convert amorphous alumina coating 30to hard ceramic α-alumina shell or sphere 40. Hard ceramic α-aluminaspheres 40 resulting from the firing to 1000° C. are shown in the SEMimage in FIG. 6. Multiple washes were conducted to remove all of thepolystyrene resulting from the dissolution of polystyrene beads 10,because the polystyrene solution tends to “gum up” the surface ofα-alumina sphere 40, precluding removal of additional polystyrene. Toseparate free-standing α-alumina spheres 40 from any extra aluminaresulting from the coating process, the fired (1000° C.) material wasplaced in water, centrifuged and filtered.

The calcination temperature of 220° C. was chosen to fit within theboundaries of the decomposition of the alumoxanes to alumina (greaterthan 180° C.) and the decomposition temperature for polystyrene (230°C.). Both values were obtained from thermogravimetric analysis (TGA)measurements. Firing to a sufficiently high temperature to form(amorphous) alumina is important because coating 30 must be sufficientlystrong to be able to withstand the toluene washing cycle. In addition,the un-fired alumoxanes are soluble in toluene so they must be renderedinsoluble before the washing stage. Conversely, if the polystyrene bead10 is allowed to pyrolize, the gases evolved burst amorphous aluminacoating 30, causing destruction to the shell. The complete removal ofthe polystyrene after the washing cycle is confirmed by TGA measurementsshowing an absence of the decomposition curve due to polystyrene.

FIG. 7 provides a summary of the pore volume and surface area for thesamples at each stage of the synthesis. The surface area of thepolystyrene beads is very low, signifying a relatively pore-freematerial. Once coated with A-alumoxane the surface area is dominated bythe alumoxane coating as indicated by a comparison with a free standingsample of the same alumoxane. Firing the coated beads to 220° C. doesnot significantly alter the pore volume and presumably allows for theremoval of the polystyrene by the toluene wash. As expected, the surfacearea decreases on sintering, although the total pore volume does notdecrease. This suggests that the final α-alumina spheres are highlyporous materials. The highly porous nature of the spheres may beutilized in various applications, such as ultrafiltration membranes,discussed below.

A further indication of the surface porosity of the alumina spheres maybe seen from SEM images. The surface of the untreated polystyrenespheres shows a smooth morphology; in contrast, the surface of thealumina spheres formed at 1000° C. is granular corresponding to theformation of alumina. The sizes of the alumina grains present(approximately 25 nm) are consistent with the size of alumina grainsformed from firing of A-alumoxane films. The thickness of the hollowalumina spheres synthesized from a 2 wt % A-alumoxane is approximately 1μm, as shown in FIG. 8. Thicker walls are formed with increasingalumoxane concentrations.

The results of hardness testing performed on the α-alumina spheres usingVicker's indention testing are shown in FIG. 9. The hardness of thehollow alumina spheres (1900±100) approached the hardness of corundum(ca. 2000) and was significantly harder than a planar piece ofA-alumoxane sintered under the same conditions. This latter observationconfirms the benefit of the spherical structure with regard to usingshape and structure to obtain a higher structural strength than isinherent in a specific material.

Hollow Mixed Metal Oxide Spheres

Hollow mixed metal oxide spheres may also be synthesized using thehollow α-alumina spheres 40 as a template. The hollow α-alumina spheres40 are covered or coated with a solution of a metal-doped alumoxane (notshown). A method of creating metal-doped materials is described in U.S.Pat. No. 6,207,130, entitled “Metal-exchanged carboxylato-alumoxanes andprocess of making metal-doped alumina”, and herein incorporated byreference. After calcining to 1000° C., the mixed metal oxide phaseforms outside of α-alumina sphere 40, resulting in a composite likeceramic bi-layer sphere. Suitable mixed metal oxides were prepared usingCa-, Er-, Mg-, Ti, and Y-doped MEEA-alumoxane to form CaAl₁₂O₁₉,Er₆Al₁₀O₂₄, MgAl₂O₄, Al₂TiO₅, and Y₃Al₅O₁₂, respectively. The formationof each phase was confirmed by x-ray diffraction measurements. Themorphology of the surface of the sphere is the same as the appropriatemetal oxide. For example, the α-Al₂O₃/CaAl₁₁O₁₈ sphere exhibitsplate-like crystals, as shown in the SEM image in FIG. 10, confirmingthe formation of hibonite. It has been previously shown in the prior artthat CaAl₁₂O₁₉, Er₆Al₁₀O₂₄, and MgAl₂O₄ are effective interphasecoatings for fiber reinforced ceramic matrix composites (FRCMCs) (see R.L. Callender and A. R. Barron, Adv. Mater. 12 (2000) 734, hereinincorporated by reference).

The hollow α-alumina and mixed metal spheres have many potentialapplications, including use in proppants, composites, and filtrationmembranes. The hardness and porosity of the hollow α-alumina spheresallow them to be used in the manufacture of proppants. The extremehardness of the spheres makes them well suited for propping openfractures in subterranean formations. The high porosity of the spheresallows the fluid in the formation to easily flow through the spherewithout significant restriction.

Hollow α-alumina spheres can also be used in the formation of ceramicmatrix composites. Fiber reinforced ceramic matrix composites (FRCMCs)are commonly employed where the performance of the ceramic matrix aloneis insufficient. In fiber-reinforced ceramics, the reinforcement isprimarily utilized to enhance the fracture toughness. The fiberreinforcement prevents catastrophic brittle failure by providingmechanisms that dissipate energy during the fracture process. Theoperation of various toughening mechanisms, such as crack deflection,fiber pull out, and fiber bridging, to a large extent depend on thedegree of chemical and/or mechanical bonding at the fiber-matrixinterface. Although spheres would not simulate pull out, they wouldprovide crack deflection. The added issue with spheres described herein,is that their density is significantly lower than the bulk ceramic,since approximately 90% of their volume is air. Thus, the overalldensity of a composite is decreased with respect to increased loading.Furthermore, the bulk dielectric constant of any body containing sphereswill be reduced with increased loading.

The inclusion of a ceramic reinforcement relies on the degree ofchemical and/or mechanical bonding at the reinforcement-matrixinterface. It is necessary to control the interfacial bond in order tooptimize the overall mechanical behavior of the composite. In thisregard, it has previously been shown that aluminate coatings on fibersprovide superior performance characteristics as compared to the nativefiber (see R. L. Callender and A. R. Barron, J. Mater. Res. 15 (2000)2228 and R. L. Callender and A. R. Barron, J. Mater. Sci. 36 (2001)4977, herein incorporated by reference). It has also been shown that notonly alumina spheres may be formed but also that multilayer spheres canbe prepared that mimic the interfacial layer often applied as a coatingon fibers to prevent deleterious chemical reactivity and provide amechanism that promotes graceful failure at the fiber-matrix interface.

Ceramic Films

Hollow α-alumina spheres, produced by the method described above, wereincorporated into a ceramic thin film formed from a 1 wt % A-alumoxaneaqueous solution. Since the 2, 5, and 8 wt % A-alumoxane solutionsproduced hollow alumina spheres with good shape retention, hollowα-alumina spheres using these concentrations were incorporated into thethin films. A flat ceramic thin film (of approximately 1 μm thicknessprepared using A-alumoxane) on a porous alumina support was used as abase for the composite structure. The surface of a flat ceramicsubstrate was brought into contact with a suspension of α-aluminaspheres in an aqueous solution of A-alumoxane solution for 2-5 seconds.The newly made thin film was dried in air overnight before firing to600° C. for 6 hours with a dwell time of 5 hours. SEM images (not shown)of the surface and cross section show the incorporation of the hollowspheres into an alumina matrix. The hardness of the sphere-reinforcedceramic matrix composites (SRCMC) as compared to the matrix material (inthe absence of the ceramic spheres) shows an improvement from 220 to 370and 650 Kg.mm⁻² after heating the composite to 600 and 1000° C.,respectively.

Composite Materials

In addition, hollow α-alumina spheres produced by the method describedabove may also be used in the formation of polymer matrix composites.Hollow α-alumina spheres were suspended in an 1:1 (wt) mixture of resin(Resin Services 302) and hardener (Resin Services 874) in an aluminumpan. A 2 wt % of the spheres to resin/hardener was used. Theresin/hardener mixture containing the spheres was cured at 50° C. for 24h. Similar composites with spheres of a nominal diameter between 50 and80 μm may be prepared in the same manner. Incorporation of the hollowα-alumina spheres into the epoxy resin results in an increase of thehardness from 170 to 570 kg/mm². SEM images of a cross section show thespheres are reasonably dispersed, however, further improvement incompatibility may be obtained by surface functionalization of theα-alumina spheres.

Ultrafiltration Membranes

Hollow α-alumina spheres produced by the method described above may alsobe utilized in the fabrication of asymmetric alumina ultrafiltrationmembranes with a hierarchical structure. As shown in FIG. 11, anasymmetric membrane 120 is comprised of relatively thin selectivemembrane 100 supported by thicker, more permeable substrate 110. Thisresults in asymmetric membrane 120 with enhanced mechanical integrityand permeability.

Before hollow α-alumina spheres may be incorporated into the asymmetricmembrane, acetic acid and methoxy(ethoxyethoxy)acetic acidfunctionalized alumina nanoparticles (A-alumoxane and MEEA-alumoxane,respectively) were prepared by methods previously described (see also R.L. Callender and A. R. Barron, “Facile synthesis of aluminum containingmixed metal oxides using doped carboxylate-alumoxane nanoparticles”, J.Am. Ceram. Soc., 83 (2000) 1777 and A. Kareiva, C. J. Harlan, D. B.MacQueen, R. Cook, and A. R. Barron, “Carboxylate substituted alumoxanesas processable precursors to transition metal-aluminum andlanthanide-aluminum mixed metal oxides: atomic scale mixing via a newtransmetalation reaction”, Chem. Mater., 8 (1996) 2331, hereinincorporated by reference). Aqueous solutions of alumoxane were thendegassed before use. Refractron™ α-alumina supports were obtained fromthe Refractron Technologies Corp. (Newark, N.J.) and were heated to 600°C. prior to use to remove surface grease. Colloidal polystyrene beads of0.75, 3, or 15 μm diameter and 3 μm spheres in the dry form, wereobtained from Polysciences, Inc.

Hollow α-alumina spheres (3 μm nominal diameter) prepared as describedabove were suspended in an aqueous solution of either A-alumoxane (1 wt%) or MEEA-alumoxane (10 wt %), prepared as described above. Thepre-formed hollow α-alumina spheres were incorporated into a ceramicmembrane formed from a 1 wt % A-alumoxane aqueous solution. Since the 2,5, and 8 wt % A-alumoxane solutions produced hollow alumina spheres withgood shape retention, spheres using these concentrations wereincorporated into the membranes. The surface of a flat ceramic membranewas brought into contact with a suspension of alumina spheres in anaqueous solution of A-alumoxane solution for 2-5 seconds. The newly madefilter was dried in air overnight before firing to 600° C. for 6 hourswith a dwell time of 5 hours. The total thickness of the membrane wasdesigned to be comparable to the flat 2 μm thick membranes previouslydescribed in C. D. Jones, M. Fidalgo, M. R. Wiesner, and A. R. Barron,“Alumina ultrafiltration membranes derived from carboxylate-alumoxanenanoparticles”, J. Membrane Sci., 193, (2001), 175-184, hereinincorporated by reference.

FIG. 12 is a schematic representation of asymmetric membrane 150. Flatceramic membrane 130, of approximately 1 μm thickness, prepared usingA-alumoxane on porous Refractron α-alumina support 135, was used as abase for macroporous membrane 140 containing hollow α-alumina spheres40. Base membrane 130 was used so as to ensure that any macroscopicholes or cracks (not shown) in macroporous membrane 140 would not leadto failure of asymmetric membrane 150. FIG. 13 provides a summary offlow, flux, and permeability characteristics for α-alumina support 135,flat 2 μm thick alumina membranes described above, and asymmetricmembranes 150.

As shown in FIG. 13, the flux and permeability parameters for themembrane with the alumina spheres derived from 5 and 8 wt % solution ofA-alumoxane show values comparable to the flat membrane In contrast, theflux and permeability for the membrane containing alumina spheresderived from 2 wt % solution of A-alumoxane is comparable to the poroussupport. The concentration of the alumoxane solution determines the wallthickness of the hollow alumina spheres. The flux and permeabilitymeasurements suggest that the thicker the wall of the pre-formed aluminasphere the more restricted the flow through the spheres and/or the flowbetween the spheres, reducing the overall available cross section of themembrane surface. As may be seen from FIG. 13, there is an inversecorrelation between the permeability and the size of the spheres.Clearly, the presence of the spheres as part of a membrane system offersa route to improved membrane performance.

An alternative route to increasing the flux and permeability ofasymmetric membranes was developed whereby polystyrene (or other polymersoluble in a solvent) beads of either 0.75, 3, or 15 μm diameter fromPolysciences, Inc., were suspended in an aqueous solution of eitherA-alumoxane (1 wt %) or methoxy(ethoxyethoxy)acetic acid alumoxane(MEEA-alumoxane, 10 wt %), prepared as described above. The surface of aRefractron™ α-alumina support was dip coated in thepolystyrene/alumoxane colloidal solution. The newly formed filter wasallowed to dry overnight before firing to 600° C. for 6 hours with adwell time of 5 hours, resulting in an asymmetric alumina membrane witha hierarchical tertiary structure. The pyrolysis/sintering temperaturewas chosen to optimize the pore size and pore size distribution of theresulting alumina membrane. As the polystyrene out-gasses duringpyrolysis, the top of the coated spheres burst. This results in amacroporous membrane in which the ceramic walls have a pore size andhence molecular weight cut-off (MWCO) defined by the alumina formed fromthe alumoxane nanoparticles rather than the macroporous structure of themembrane itself. The permeabilities of these membranes are equivalent orbetter than the support. One advantage of this method is that themembrane is produced in one step, without having to fabricate the hollowalumina spheres separately. However, one disadvantage is that only oneceramic material may be used in the fabrication of the membrane.

A schematic representation of resulting asymmetric membrane 250 producedby this method is shown in FIG. 14. Flat ceramic membrane 230, ofapproximately 1 μm thickness, prepared using A-alumoxane on porousRefractron α-alumina support 235, was used as a base for macroporousmembrane 240 containing polystyrene beads (not shown). As describedabove, the polystyrene out-gasses during pyrolysis leaving hollow voids45, and some of the coated spheres burst, leaving concave indentions 47in the surface. Base membrane 230 was used so as to ensure that anymacroscopic holes or cracks (not shown) in macroporous membrane 240would not lead to failure of asymmetric membrane 250. The identity ofthe alumoxane, the concentration of alumoxane solution, and the diameterof the polystyrene were investigated to determine their effects of themembrane structure and the flow/flux performance of the membrane. Atomicforce microscopy (AFM) of the colloids deposited in the alumoxane filmsshows that there is a difference in spacing between the spheresdepending on the identity of the alumoxane solution they are dispersedin. When mixed with MEEA-alumoxane, the spheres appear to be touching,however, A-alumoxane results in the spheres separated by about 4 nm.MEEA-alumoxanes provided a higher ceramic yield and smaller average poresize than A-alumoxanes. The use of higher concentrations of alumoxanes(10 wt %) was found to be detrimental for the optimization of the fluxand permeability due to the thickness of the final membrane. Althoughthe 10 wt % concentration of A-alumoxane allowed for the colloidal beadsto align on the surface of the membrane support, it prohibits the outgassing of the decomposition products of the polystyrene by insulatingthe colloids (as shown in the SEM image in FIG. 15). The resultingmembranes were similar in appearance to those described above that wereformed using the preformed alumina spheres. In contrast, the use of 10wt % MEEA-alumoxane solutions allowed for alignment of the polymerbeads, and resulted in the rupture of the ceramic spheres upon pyrolysisto give a macroporous high surface area membrane, as shown in the SEMimage in FIG. 16. MEEA-alumoxane has a lower ceramic yield (37%) thanA-alumoxane (76%) and therefore results in a thinner more porous coatingof the polymer beads that allows the volatiles to out-gas.

Even though 10 wt % MEEA-alumoxane solutions produced a membrane withthe desired macroscopic features, the average pore size of a membranederived from this alumoxane is larger (10 nm) and has a large pore sizedistribution (5-30 nm) than membranes formed from A-alumoxane. Since thethickness of a flat membrane derived from 10 wt % MEEA-alumoxane wasfound to be similar to those made from a 1 wt % A-alumoxane, the lattermay be used to provide uniformly small pore size and pore sizedistributions, shown in the SEM image in FIG. 17. The use of A-alumoxaneresults in a pore size of 7 nm as determined from BET measurements(defined below). The MWCO of the filter prepared using the A-alumoxanesgave an 80% rejection of molecular weights of between 9,000 and 10,000g.mol⁻¹, corresponding to a pore diameters of >4 nm. Since the basemembrane was prepared from A-alumoxane it is not as important tomaintain the pore size of the membrane with the macroporous featureswith regard to molecular weight cut-off performance. A comparison offlux measurements will change by altering the alumoxane.

As previously described, when the products from the pyrolysis of thepolystyrene out-gas, the top of the coated spheres burst, resulting in amacroporous membrane in which the ceramic walls have a pore size definedby the alumoxane. The resulting “divots” resemble a honeycomb pattern,as shown in the SEM image in FIG. 18. An oblique view of these surfacecraters is shown in the SEM image in FIG. 19.

The structured array of the macroporous structured membrane iscontrolled by the size of the polystyrene beads. Membranes formed withthe 0.75 μm polystyrene beads produced an evenly distributed honeycombarray. The macropores are spaced 100 nm apart, with the shell wallthickness corresponding to 1 μm. As the template diameter increases(i.e., larger polystyrene beads are used) the regularity of the arraydecreases. Presumably this is due to either the ease of packing or thedecreased quantity of beads per volume (of solution) for the largerbeads. Regardless of polystyrene bead size, the final ceramic wallthickness remains constant and is defined by the identity andconcentration of the alumoxane solution.

FIG. 20 summarizes the flux and permeabilities achieved with the variedmacro featured membranes. The flow, flux, and permeability are thehighest for the membrane with the largest macroporous features. Thesemembranes also exhibit the largest increase in the surface area. Themembranes derived from MEEA-alumoxane showed higher flow rates thanthose derived from A-alumoxane due to the larger average pore size (10nm) and broader pore size distribution (5-30 nm) of the MEEA-alumoxanederived macroporous features.

The efficiency, such as flow and permeability, of an ultrafiltrationmembrane can be improved by developing a hierarchical membrane, byincreasing the surface area. This is of importance becauseultrafiltration membranes have pore sizes down to 2 nm. Pore sizes ofthis diameter will inhibit the flow of a solution through the filter.Two methods to increase the surface area have been investigated,depositing hollow spheres in the membrane, forming a convex shapedmembrane, or templating the membrane with polystyrene micro spheresforming a macroporous, concave, featured membrane. In both of thesecases, the flow through the membrane increased. However, for the hollowspheres deposited into the membrane, those with the thinnest shell,increased the flow, compared to the “flat” membrane because the surfacearea has doubled. As the shell thickness increased, the flow through themembrane slowed to values less than the “flat” alumina membrane. Themacroporous templated membrane also increased the flow through themembrane by doubling the surface area of the membrane. However, the flowcannot exceed the flow of the support, therefore, it is desirable forthe values to approach the support values as close as possible.

The above discussion and Figures are meant to be illustrative of theprinciples and various embodiments of the present invention. Numerousvariations and modifications will become apparent to those skilled inthe art once the above disclosure is fully appreciated. It is intendedthat the following claims be interpreted to embrace all such variationsand modifications. For example, the term “sphere” should not beinterpreted to include only bodies that appear completely circular inall views, but to include bodies that are generally round. Sequentialrecitation of steps in the claims is not intended to require that thesteps be performed sequentially, or that one step be completed beforecommencement of another step.

Measurement Apparatus and Techniques

SEM studies were performed on a Phillips XL-30 ESEM scanning microscope.The samples were attached to a metal mount using carbon tape. Due to theinsulating nature of the materials, a thin layer of gold was applied asa coating to provide a conducting surface. Micro-indention testing wasperformed on a Micromet microhardness tester with a standard diamondtip. The size of the indentation (10 μm) is smaller than the size of thespheres and all indentions were made so as to minimize the effects ofthe curvature of the sphere. Hardness was determined by inserting theload weight and the area of indention into the Vicker's equation:H_(v)=1.85444(P/d²) where P is the load in kg and d² is the area ofindention in mm². Five indentation measurements were performed on eachsample with a loading time of 10 seconds per measurement. Powder X-raydiffraction patterns of A-alumoxane and metal-doped MEEA-alumoxane weredetermined by using a Siemens Diffractometer, with a scan area of 20-80degrees, step size of 0.1 degrees, and count time of 10 seconds.Porosity, surface area, and pore volume were obtained using nitrogenadsorption/desorption techniques using a Coulter™ SA3100™. Helium wasused to determine the free space in the sample tube and nitrogen as theabsorbate gas. All samples were outgassed at 300° C. for 2 hours under astream of dry nitrogen using a Coulter™ SAPrep™. Calculations were basedon the cross sectional area of nitrogen using the value of 0.162 nm².Surface area was calculated using the BET (Brunauer, Emmett and Teller)equation with 5 data points. Pore volume calculation was performed at arelative pressure of 0.9814. Thermogravimetric/differential thermalanalyses were obtained on a Seiko 200 TG/DTA instrument using a carriergas of either dry nitrogen or air.

AFM images, grain size analysis, and surface roughness analysis ofsamples were obtained using a Nanoscope IIIa Scanning Probe Microscope(Digital Instruments, Santa Barbara, Calif.) in tapping mode. FESP tipswere used with a pyramidal shape and end radius of 5-10 nm (DigitalInstruments). Samples were attached to 15 mm magnetic specimen diskswith carbon tape. Roughness and cross-section analysis were determinedby the accompanying Nanoscope IIIa software.

Porosity, surface area, pore volume, and pore size distributions wereobtained using a Coulter SA3100. Helium was used to determine the freespace in the sample tube and nitrogen as the absorbate gas. All sampleswere out gassed at 300° C. for 2 hours under a stream of dry nitrogenusing a Coulter SAPrep. Calculations were based on the cross sectionalarea of nitrogen using the value of 0.162 nm². Surface area wascalculated using the BET (Brunauer, Emmett and Teller) equation with 5data points. Pore size distributions were determined using the BJH(Barrett, Joyner, and Halenda) technique using 65 data points from thenitrogen adsorption isotherm (see S. J. Gregg, K. S. W. Sing,Adsorption, Surface Area and Porosity, 2nd Edition, Academic Press,London (1982), herein incorporated by reference). Pore volumecalculation was performed at a relative pressure of 0.9814.

Pure water flux was measured on both the carboxylate-alumoxane derivedfilters and the hierarchical carboxylate-alumoxane derived filters at apH close to the isoelectric point of alumina. Samples were placed in aNalgene, model 300-4000 dead end filtration cell. A zero air tank wasconnected to the cell for pressure, and a regulator was used to maintaina constant pressure of 10 psi. Permeate was collected at atmosphericpressure, so that the pressure at the end of the regulator was equal tothe transmembrane pressure. Permeate volume was measured over time tocalculate flux and permeability.

While various preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the scope of the invention. The examplesdescribed herein are merely illustrative, and are not limiting. Forexample, various polymers, carboxylate-alumoxanes, and metal dopants canbe used. Likewise, the solvents, washes, and temperatures and pressuresof the processing steps can be varied, so long as the desiredcomposition is formed. Accordingly, the scope of protection is notlimited by the description set out above, but is only limited by theclaims that follow and includes all equivalents of the subject matter ofthe claims. In any method claim, the recitation of steps in a particularorder is not intended to limit the scope of the claim to the performanceof the steps in that order, or to require completion of one step priorto commencement of another step, unless so stated in the claim. Forexample, it will be understood that the drying and firing can beaccomplished in a single process step and such an embodiment is intendedto be within the scope of the present claims.

1. A method comprising propping open fractures in a subterraneanformation with hollow, porous spheres of alumina or aluminate.
 2. Themethod of claim 1 wherein the spheres have a hardness of at least 750kg/mm² on the Vickers hardness scale.
 3. The method of claim 1 whereinthe spheres have a hardness of at least 1800 kg/mm² on the Vickershardness scale.
 4. The method of claim 1 wherein the spheres range from1-80 microns in diameter.
 5. The method of claim 2 wherein the spheresrange from 1-80 microns in diameter.
 6. The method of claim 3 whereinthe spheres range from 1-80 microns in diameter.
 7. The method of claim1 wherein all or a portion of the spheres are contained within a polymermatrix composite.
 8. The method of claim 7 wherein the composite is areaction product of a resin and a hardener.
 9. The method of claim 7wherein the composite is an epoxy resin.
 10. The method of claim 7wherein the composite has a hardness greater than the hardness of thespheres.
 11. The method of claim 7 wherein the spheres are surfacefunctionalized.
 12. The method of claim 1 wherein all or a portion ofthe spheres further comprise metal oxide.
 13. The method of claim 7wherein all or a portion of the spheres further comprise metal oxide.14. The method of claim 1 wherein the spheres have a thickness of about1 micron.
 15. The method of claim 1 wherein the spheres aremultilayered.
 16. The method of claim 1 further comprising flowing fluidfrom the formation through the pores of the spheres.
 17. A wellborefracture proppant comprising hollow, porous spheres of alumina oraluminate.
 18. The proppant of claim 17 wherein the spheres have ahardness of at least 750 kg/mm² on the Vickers hardness scale.
 19. Theproppant of claim 17 wherein the spheres have a hardness of at least1800 kg/mm² on the Vickers hardness scale.
 20. The proppant of claim 17wherein the spheres range from 1-80 microns in diameter.